technical training associates presents 20 hvacr...

Technical Training Associates

Presents

20 HVACR Troubleshooting Problems

Volume 2

By Jim Johnson

A Practical Approach To Troubleshooting HVACR Equipment

20 HVACR Troubleshooting Problems Volume 2 By Jim Johnson

Copyright © 2011 Technical Training Associates

HC 70 Box 3172

Sahuarita, AZ 85629

(520) 625-6847

www.techtrainassoc.com

All digital copyrights reserved. Permission is granted by the publisher to print and

photocopy the material herein for the purpose of conducting individual training

sessions. Distribution of hard copy versions to the public for sale, or digital

copying of any and all information contained herein is prohibited.

This troubleshooting handbook contains 20 separate troubleshooting scenarios

designed to test the skills of HVAC R technicians. In each situation, all the

information necessary to arrive at a diagnosis is provided.

The solutions to the problems are presented in a separate section at the end of the

book.

If you are working individually with this handbook, use it to test yourself. Review

the problem and any related graphics or wiring diagrams, then, record your

diagnosis before checking your answer in the solutions section.

If you are a serviced manager conducting in-house training sessions, or an

instructor facilitating a classroom or lab training exercise, you may print and

photocopy the problems and diagrams for classroom distribution. Digital copying

of this CD or electronic file is prohibited as noted in the copyright notice above.

http://www.techtrainassoc.com/

Troubleshooting Problem #1

A SPLIT SYSTEM THAT‟S NOT KEEPING THE BUILDING

COMFORTABLE

We‟ll begin our explanation of this problem by saying there is a lot of

history with the seven-year-old residential system in this three bedroom, two bath

rental home, and the several technicians who have been there ahead of you have

suggested that the problem with this under-performing system could be a system

undercharge, kinked tubing in the wall, or a “slightly inefficient compressor”, and

all of the diagnosis up to this point has been accomplished by working with the

condensing unit only.

When you arrive, you decide to take a different approach to evaluating the

performance of this system and focus on the air handler and indoor coil in the attic.

The first thing you notice is that the methods employed in this installation make it

extremely difficult to access the “A” coil, but you are able to make the necessary

gauge connections we‟re showing in Figure One (the HP hose for a High Pressure

reading at the air entering the coil, and the LP hose for a Low Pressure reading at

the air leaving the coil) to test the pressure drop across the coil.

Troubleshooting Problem #1 Figure One

After monitoring the operation of the system for several minutes you note

that your pressure test shows the drop being measured is 0.6 in. w.g.

Your troubleshooting question: What is the next step you need to take in

order to continue your evaluation of this equipment?


A HEAT PUMP THAT WON‟T COOL

In this troubleshooting situation, the equipment you‟re servicing is a 240-

Volt, single-phase split system, and the customer‟s complaint is simply that there is

no cooling. They also tell the dispatcher that they had the same problem at the

beginning of the cooling season last year, at which time a part was replaced and the

unit operated OK until now. When you arrive, you find the while the indoor and

outdoor fan motors are operating, the root of the problem is that the compressor is

attempting to start, but kicks off on its overload. When you remove the access

panel, you check the schematic and note the wiring configuration for this

compressor, shown in Figure One.


However, when you survey the equipment, expecting to find the two run

capacitors (Ca and Cb) wired in parallel (meaning that their value will be added to

provide the proper PSC circuit for the operation of the compressor, while only the

Ca capacitor will be used in the off cycle to provide a trickle circuit through the

start winding), you discover that the wiring has been modified and the compressor

circuit now only employs one run capacitor as we‟re showing in Figure Two on the

next page.

Your troubleshooting question: What is the specific cause of this

compressor‟s failure to start?

Troubleshooting Problem #2 Figure Two


A ROOFTOP UNIT THAT WON‟T SHUT DOWN

In this troubleshooting situation, it‟s winter, and our customer has called to

say that the older package unit that heats and cools their office is managing to

warm up the area, but then seems to cool down too much before providing heat

again. When you arrive, the first thing you note is that the indoor blower motor is

operating, and that the fan switch on the thermostat assembly is set in the position

we‟re showing in the diagram in Figure One on the next page.

You also note that:

….the discharge air temperature is what you would expect it to be for this gas-fired

heating system.

…. turning the thermostat to a lower set point and waiting a reasonable time proves

that the indoor blower motor doesn‟t shut down.

When you access the unit‟s electrical system, you note that the burners are

operating normally since the thermostat is again calling for heat, and you begin

your troubleshooting procedure by placing a voltmeter at H1 and H2 on the IBDR

coil and confirm 24 VAC. When you instruct the customer to turn the thermostat

down to a lower set-point, you note that the burners shut down, and after a delay,

your voltage reading at IBDR is 0 VAC, while the indoor blower motor continues

to operate. Your next voltage test is at #2 and #4, M1 and M2, and your reading is

0 VAC.

Your troubleshooting question: What is your next step in solving this

problem?


A SECOND OPINION ON AN UNDERCOUNTER UNIT

This troubleshooting situation involves a small, two-door side-by-side unit that

allows for beverage storage in the right side compartment, and a small quantity of

frozen items on the left side compartment. The customer‟s original request for

service reported that both sections of the unit were warm.

This unit is compact, built-in under the counter in a tavern, and difficult to

access. The first technician to respond to this situation told the owner that he was

familiar with common failures of this unique equipment, and after a quick look and

observation of the operation of the unit (no meter tests), he explained to the

customer that the compressor needed to be replaced and that he would have to

order the necessary parts.

After the technician left, the customer called the office to request a second

opinion. When you arrive, you also determine that this unit employs a PSC-

operated hermetic compressor, fan-cooled condenser, and an evaporator fan motor

and defrost system to maintain temperature and air flow. And, like the first

technician, you determine quickly that air is circulating both inside and under the

cabinet, and that the compressor isn‟t starting. In Figure One on the next page,

we‟re showing you the basic compressor wiring configuration you see after making

sure the power supply is off, removing the necessary access panels, setting up a

work light, and removing the cover from the start components.

Since you find no visual evidence of wiring or component failure, you

remove the relay and external overload, and use an ohmmeter to test the

compressor (which is at room temperature) at the terminals shown in Figure Two.

Your results are:

1. From C to S, 6 Ohms.

2. From C to R, Infinity.

Your troubleshooting question: Have you confirmed the first technician‟s

diagnosis, or will you be advising the customer that the repair procedures will

change?


A MOTEL ROOM HEAT PUMP THAT‟S NOT HEATING

This troubleshooting problem is focused on one of those relatively rare situations

where you‟re called in to service a wall-through unit. The report from maintenance

personnel at the small motel is that the equipment is “blowing cool air when it‟s

supposed to be heating”. The electrical profile of this unit (shown in Figure One on

the next page) is as follows:

1. A heat pump with a three-speed blower motor

2. Operating voltage is 230-VAC

3. A dual capacitor serves the PSC motors in the equipment.

When you arrive, you find the following:

1. The unit is delivering cool air and the room temperature is 65 degrees.

2. The blower motor and compressor are both operating.

3. The controls are set for high fan speed operation and the heating mode.

After disconnecting the power supply and removing the wrapper, your visual

inspection shows no broken wires, and you reconnect the power supply to perform

voltage checks. Your first check at RV shows 0-VAC and your subsequent tests at

both the N.O. and N.C. connections on the defrost control also show 230-VAC.

Your troubleshooting question: What is the next step you need to take in

repairing this equipment?


A HEAT PUMP THAT‟S NOT COOLING PROPERLY

Heat pumps, because of their ability to heat and cool an area, are sometimes

referred to as a reverse cycle refrigeration systems. And, in this troubleshooting

situation, the problem is related to the refrigeration system, not the electrical or air

handling system. Our particular heat pump (see the illustration in Figure One) is a

long way from being new, but our customer wants a full assessment of what it

might take to get the unit operating properly again.


As you can see in our illustration, this unit employs two separate metering

devices and two check valves to accomplish the necessary patterns of refrigerant

flow according to the cycle selected by the customer. We‟re showing the proper

refrigerant flow with the unit in the cooling mode.

Our problem, as it‟s reported by our customer, is that the unit “worked fine

during the heating season” but didn‟t cool properly the first time they turned it on

for cooling. When you arrive, here‟s what you find:

1. The compressor, outdoor fan motor and indoor fan motor are running, but

there is little cooling taking place.

2. The suction pressure is high.

3. Liquid subcooling is zero.

4. Liquid refrigerant is flooding back to the reversing valve, accumulator, and

compressor.

Your troubleshooting question: Which refrigeration system component is the

source of the problem?


A SODA VENDING MACHINE THAT‟S NOT PERFORMING

PROPERLY…SOMETIMES

Author’s note….Your “hint” relative to this particular troubleshooting problem is

to remember the fundamental concept of maintaining a pressure differential in a

vapor compression system.

The equipment in this troubleshooting problem has basically the same

refrigeration system found on a standard refrigerator…a capillary tube metering

device is used to feed refrigerant to the evaporator. The other „refrigerator‟

similarities are a fan-cooled condenser, and a forced-air evaporator. Figure One

shows you an illustration of the evaporator section of the system (top) and he

condenser section located in the lowest part of the cabinet.


The complaint from the customer, who has positioned the unit outside their

small motel, is that while the soda seems to be properly cooled during the middle

of the day, there are times when the product is warmer than it should be.

Temperatures during the day are in the mid 70s, while the temperature later in the

day drops to the low 40s. When you make your initial stop early in the afternoon,

you note that the unit seems to be operating normally with both the evaporator and

condenser fan motors running normally. And, when you check the wiring diagram,

you note the condenser fan wiring shown in Figure Two.


Since your initial check of the equipment shows no problems, you decide to

schedule a follow-up trip back to further evaluate the situation when the outside

temperature has dropped. When you return during the low-ambient situation, you

note again that both the evaporator fan motor and condenser fan motor operate

normally.

Your troubleshooting question: What is the next step you need to take to

resolve the “not cooling enough” problem with this equipment?


A SPLIT SYSTEM THAT‟S NOT COOLING

In this troubleshooting situation the equipment is a split system that‟s

approximately 5 years old, consisting of a gas furnace that provides heat in the

winter, and employs a condensing unit and “A” coil to provide summer cooling.

There are actually three questions to answer in this problem….two of them

regarding overall troubleshooting procedure, and the third one regarding the

necessary repair.


The information you get from the dispatcher is that the customer‟s

description of the complaint is “blowing warm air”. Which brings us to the first

question about this problem: What are two components located in the furnace

assembly that can be eliminated from consideration as the possible source of the

problem?

When you arrive, you confirm that the system is blowing warm air. You also

confirm that the thermostat is set to the cooling mode with the set-point below

room temperature, and the fan switch is set to the AUTO position. A check on the

condensing unit reveals that the condenser fan motor is operating normally, but the

compressor is not running. Your second question: What two components inside the

condensing unit can be eliminated from consideration as the source of the

problem?

After turning off the disconnect switch and removing the access cover, you

find that the compressor is hot to the touch, indicating it is attempting to start.

Based on that observation, you disconnect all the appropriate wiring when

necessary, and, with a digital meter, check the start components shown in Figure

One. And, you get the following results:

1. At terminals SC, you read the microfarad rating shown on the capacitor case.

2. At terminals RC, you read the microfarad rating shown on the capacitor

case.

3. At the SR terminals of the potential relay coil, infinity.

4. At the SR terminals of the potential relay switch, a fraction of an ohm.

Your final troubleshooting question: What do you have to do to get this unit

operating again?


A 90-DEGREE OUTDOOR TEMPERATURE AND A CUSTOMER THAT‟S

HOT UNDER THE COLLAR

In this troubleshooting situation, it‟s the middle of summer, and our “hot

under the collar” customer is hot for two reasons: The temperature in their house is

uncomfortable, and you‟re following another technician who was there less than a

month ago in response to the same complaint, which is that the system is “blowing

warm air.”


When you arrive, you find the following conditions:

1. The outdoor temperature is 90-degrees.

2. The indoor fan motor is running.

3. The thermostat is set in the cooling mode at a set-point of 70-degrees, with

the fan switch in the On position…because the customer thought for some

reason that this fan switch setting would help the situation.

4. The indoor temperature (and the customer, as we mentioned earlier) is

uncomfortable.

While you‟re assessing the situation, the customer tells you that they had the

equipment (it‟s a package unit) serviced less than a month ago for the same

complaint of “no cooling”. You turn re-set the fan operation on the thermostat to

the AUTO position, and note that the indoor fan motor continues to run normally,

and, since you‟ve done all you can do on the inside, you head for the roof.

When you get near the unit, you note that the condenser fan motor isn‟t

running. You also note upon a closer inspection that the compressor, which isn‟t

overly hot to the touch, but isn‟t cool either, isn‟t running at the moment. While

checking the wiring diagram shown in Figure One….we‟ve eliminated the heating

wiring and electrical components to simplify the diagram….you note that the

compressor and condenser fan motor both start, run briefly, then shut down again.

Since you note that this unit is equipped with two protection switches on the

control voltage segment of the diagram, you decide to connect your gauges to the

high and low side access valves. When you do, you find that when the unit starts

again, the low side pressure in this R-22, capillary tube metering device system

quickly drops down just below 40 PSIG, and the compressor and condenser fan

motor again shut down. After a brief wait, you note that this cycle repeats.

Your troubleshooting question: What is the problem with this unit and what do

you need to do in order to perform a permanent repair?


A FOLLOW UP SERVICE CALL ON A THREE-PHASE UNIT

In this troubleshooting situation, in which a customer‟s 7 ½ ton, three-phase

(compressor and indoor fan motor, that is) package unit “isn‟t cooling like it

should”, Technician Number One gets the call and responds. Upon his arrival, he

finds the unit not running at all. Following standard troubleshooting procedures, he

begins with the fused disconnect, a representative diagram of which is shown in

Figure One.


Using a voltmeter, he is able to measure 230-volts at the line side between

L1 and L2, between L1 and L3, and between L2 and L3. On the load side of the

disconnect assembly, he measures 230-volts between L1 and L2, but not between

L2 and L3, nor between L1 and L3. Closely inspecting the disconnect switch, he

determines the probable reason for the blown fuse. There is discoloration on the L3

connecting lug and knife blade switching assembly. Advised by the customer that

there have been several trips to replace a blown fuse in this unit, he replaces the

entire disconnect switch and all three fuses, which allows the unit to start operating

again.

The next day, a call comes through the dispatcher that the same customer is

calling to say that the unit “isn‟t cooling again” and, since you‟re closer to the

location, you pick up the call. When you arrive and go to the rooftop, you find the

following:

1. Compressor and outdoor fan motor are running

2. A frost build-up on the indoor coil.

3. The indoor blower motor (see Figure Two) running counter-clockwise.

4. The suction line is sweating and frosted.


Your troubleshooting question: What error did Technician Number One

make, and what do you need to do in order to get the unit running normally again?


A REFRIGERATION SYSTEM WITH A HISTORY OF COMPRESSOR

FAILURE

In this troubleshooting situation, it is a walk-in refrigeration system in a busy

restaurant that has experienced a compressor failure…specifically a mechanical

failure. And it‟s not the first time. This particular unit has experienced compressor

failure twice in the several years it‟s been in operation, and the customer is looking

for some assurance that history will not repeat itself after you have completed your

compressor replacement. In Figure One, we‟re showing you an overview of this

particular system, illustrating the actual physical layout of the system components

and their relationship to each other.

Troubleshooting Problem #11 Figure one

And, in Figure Two, we‟re showing you a more detailed look at one segment

of the system: The TXV, evaporator coil, and suction line. To consider the correct

answer to this problem, look at both illustrations from the perspective of something

that is not shown as part of the system, and would be on the liquid line if it was

there.


Your troubleshooting question: What is the source of problem that is responsible

for repeated compressor failures on this unit?


A HEAT PUMP WITH AN “AIR FLOW” PROBLEM

This troubleshooting situation centers on a customer who has called to say

that their heat pump isn‟t working properly in the heating mode. Their description

of the problem is that they “never feel any air” when they select the heating mode.


Upon your arrival, you find the following conditions:

1. Thermostat turned off. (The customer thought it best to do that.)

2. Indoor temperature at 70 degrees.

When you set the thermostat system switch to the heating mode and make sure

that the set-point is above room temperature, you note that there is no indication of

air flow into the central return grille, which is nearby in the ceiling, just below this

rooftop package unit. What the customer called a “no air flow” problem, is, in

reality, a “no run” condition. When you switch the system switch to the cooling

mode and choose a set-point below room temperature, you note normal air flow

into the return.

With the thermostat is set back into the heating mode, you head to the roof and

remove the access panel. Using a voltmeter to accomplish the appropriate tests that

yield the following results: (Note the wiring diagram in Figure One)

1. 24-volts at 4R

2. 24 volts at 4R2

3. 240-volts at 4R1

Your two-part troubleshooting question: Which component has failed, and why

does the unit operate in the cooling mode, but not the heating mode?


AN ELECTRIC FURNACE THAT‟S NOT HEATING UP

The customer‟s description of this troubleshooting problem is that the unit is

“blowing cool air.”


This particular unit employs five heating elements that are energized in a

step fashion via five separate sequencers, as shown in Figure One.

Each individual element is protected by a fusible link (shown to the left of

each element) and an automatic re-set, over-temperature device (shown at the right

of each element). Each sequencer has a Main (M) set of N.O. ….Normally

Open…contacts, as well as an Auxiliary (A) set. The method of controlling each

sequencer is applying 24-volts to its control section, closing the M contacts first on

a time delay, then closing the A contacts on a subsequent delay.

Upon your arrival, you find the thermostat turned off and the system sitting

idle. Your first action is to turn the thermostat to the HEAT mode and leave the fan

switch in the AUTO position. After a slight delay, you note that the air handler

starts and operates at low speed. After allowing a reasonable time for the discharge

air temperature from a supply register to warm up, you note only a very slight rise

in temperature.

With the access panel removed, you use both an ammeter and a voltmeter to

evaluate the operation of the electrical system. With an ammeter clamped around

the L1 leg of element #1 (controlled by SEQ#1) you note a current draw of

approximately 15 amps. You also note a current draw of 0 amps when checking the

circuits of elements 2, 3, 4, and 5. With a voltmeter, you confirm the following

relative to sequencer #1:

1. 24-volts at the control section.

2. 0-volts at M1 and M2.

3. 24-volts at A1 and A2.

Your two-part troubleshooting question: Why is the unit heating air only

slightly, and which component needs to be replaced?


A GAS FURNACE THAT‟S NOT HEATING UP

In this troubleshooting situation the customer is complaining of “no heat at all” and

the unit is an induced draft gas furnace that employs an electronic control system.


When you arrive at the customer‟s home you find the unit turned off at the

thermostat. The room temperature is below 65-degrees, and when you initiate a

cycle, you note:

1. The IDM (Induced Draft Motor) stars and runs for about 15 seconds

before…

2. The HSI (Hot Surface Igniter) glows and draws the correct level of

current.

3. There is no burner ignition.

4. The igniter stops glowing.

5. The IDM shuts down.

After consulting the diagram in Figure One, you initiate a cycle again, this time

performing the following control voltage checks and getting the following results

as ignition is attempted. (Note: TP on this diagram denotes Test Point….TP3, 4

and 5 are operating voltage test points and TP2, 6 and 7 are control voltage test

points).

1. 24-volts at SEC2 and TP6

2. 24-volts at SEC 2 and TP7

Your troubleshooting question: What is the next step you need to take in order

get this furnace back on line?


A HEAT PUMP THAT‟S NOT PERFORMING AT FULL CAPACITY

The equipment in this troubleshooting problem is a heat pump that has a four-stage

heating system, and the customer is complaining that the unit isn‟t heating enough

when the temperature drops at night. This is a three-phase, 220-volt heat pump in

which the first stage of heating is the refrigeration system, and stages 2, 3 and 4 are

three 5KW auxiliary heating elements.


The electrical system is shown in two illustrations, with the operating voltage in

Figure One, and the control voltage, as well as the operating voltage for the

auxiliary heat system, shown in Figure Two.


When you arrive during the day to follow up on this problem, the

temperature is warmer than the night-time temperatures have been, and the unit is

operating normally in the heating mode, with stage one of the heating system

operating normally and maintaining comfort in the home.

To do a complete evaluation of the system, you take the necessary steps to

initiate the 2nd

, 3rd

, and 4th stages of heat, and using a voltmeter and ammeter, you

find the following:

1. 220-volts applied to 1H, 2H, and 3H. (See Figure Two)

2. The manufacturer‟s recommended current draw at point #31.

3. A current draw of zero at point #33 and #35.

Your troubleshooting question: What specific repair do you have to accomplish

in order to bring this unit back to its full operating capacity?


A GAS FURNACE THAT‟S HEATING INTERMITTENTLY

The customer in this particular troubleshooting situation is somewhat savvy

about their 12-year-old gas furnace, so when they call for help, they tell you that

when it got very cold in the house, they disconnected the power supply to the unit,

and then, after turning the 120-volt service switch back on, the operation seemed

normal and the house got warm “for a day or so”. But the failure repeated, so they

called for service.

When you arrive, you find the furnace running and the indoor temperature at

an acceptable level. Your evaluation of the equipment includes checking the flame

sensor (OK), checking the drain trap, and checking the fuel pressure at both the

inlet and the outlet of the gas valve (also OK). As your next step, you go through

the re-set procedure by disconnecting the power supply for the appropriate time,

and turning the service switch back on, expecting the blower to run for 30-seconds

on a system re-start since that‟s considered normal operation for this particular unit

after a power interruption.


But the blower doesn‟t start until you hand-spin the squirrel cage. At this

point, you‟re leaning toward the diagnosis that the motor is failing (see the partial

diagram in Figure One), so you double-check two things: A test at the motor leads

from the printed circuit board shows 120-volts, and a check of the run capacitor

with your digital meter shows a reading of 2.5 microfarads.

Your troubleshooting question: Which component needs to be replaced?


SIX HOUSES WITH THE SAME COMPLAINT

In this troubleshooting situation you‟ve been asked to investigate a problem

that‟s been ongoing since the gas furnaces were installed in six three-bedroom

homes. The compliant regarding each of them is the same: The back bedroom

doesn‟t get enough heat.

When you arrive, you find that several attempts have been made to solve the

problem. Various supply registers have been closed or nearly closed altogether,

and some have cardboard taped to them in an effort to get more air flow to the

supply registers in the back bedroom in the duct system, which is the last branch in

the extended plenum system shown in Figure One.


In preparation for a test of the duct system, you remove the cardboard and

duct tape, and adjust the supply registers to their proper opening in order to provide

supply air to the building in a balanced manner.

When you initiate a call for heat, you observe that the ignition system

functions and the main burner lights and burns correctly, and the 4-speed blower

motor operates on low speed. You also note that the equipment tag shows that the

temperature rise of this furnace should be 65-degrees. When you perform a

temperature rise test by checking the return air and supply air temperature you find

that the return temperature is 70-degrees and the supply temperature is 140-

degrees.

Your troubleshooting question: What installation error was committed that

prevented these furnaces from properly serving the back bedrooms?


THE EXCESSIVELY EXPENSIVE HEAT PUMP

Your role in this troubleshooting situation is a follow-up to visits by other

technicians who responded to the customer‟s complaint about the cost of operating

their 12-year-old heat pump in the winter. Opinions offered so far range from a

possibly failing reversing valve to improper use by the customer.


This customer has been comparing their electric bills from several previous

years and they are convinced that the heat pump is the source of their increased

utility cost. When you arrive, you find a heat pump that employs a supplemental

heat package and uses an air switch for defrost control. The diagram in Figure one

shows you a schematic representation of the refrigeration system, and the two-

element supplemental heat system, Figure Two shows your legend.


When you monitor the operation of the unit, all indications are that the

refrigeration system is properly charged and operating normally, and your

temperature differential test at the reversing valve shows less than a 2-degree

difference on all inlet and outlet tubing connections. When you initiate a defrost

mode, you note that the outdoor fan motor continues to run. Testing with a

voltmeter at the appropriate test points, you get the following results:

1. At the coil of the DFR, 240-volts.

2. At contacts 1 and 2 of the DFR, 240-volts.

3. At contacts 4 and 5 of the DFR, 0-volts.

Your troubleshooting question: What is the specific failure that is causing the

higher-than-normal operating costs?


A NEARLY ANCIENT PACKAGE UNIT THAT‟S NOT COOLING LIKE IT

SHOULD

The description of the equipment in this troubleshooting problem is form the

customer, and while they‟re not sure of the age of their gas back, they are correct

in their observation that it‟s not keeping their house comfortable.


When you arrive, you find that this unit is one that has been around a while,

employing an early generation printed circuit board system (see the illustration in

Figure One) that has two electromechanical relays to control the operation of the

blower motor at the appropriate speed for the heating and cooling modes.

And it‟s the blower motor that is the focus of your attention in this situation,

because once you have checked to make sure that the thermostat is set in the

cooling mode and turned down below room temperature, you find no air coming

from the supply registers. A trip to the roof reveals the following conditions:

1. The PSC Indoor Fan Motor (IFM) with its IFMRC (Indoor Fan Motor Run

Capacitor) is not running.

2. The Compressor (COMP) and Outdoor Fan Motor (OFM) are running.

3. The suction line is cold and sweating.

Consulting the schematic diagram and locating connections for the common

and high speed operation of the IFM, you note a reading of 0-volts. Testing at the

Cooling Indoor Fan Relay (CIFR) normally-open contacts, you read 230-VAC, and

testing at the CIFR coil, your meter shows 24-VAC.

Your troubleshooting question: Which component needs to be replaced in order

to get this unit operating again?


A WALK-IN FREEZER THAT‟S OPERATING ERATICALLY

In this troubleshooting situation you are dealing with equipment that has

only been in operation for two months. And during that time frame, the customer

has complained that at times the box temperature is higher than it should be, but

the unit them seems to recover and freeze OK until the next time some of the items

inside are found to be thawing.


The details on this freezer are as follows:

…The refrigeration system was field-designed and assembled with components

purchased individually, and the condensing unit is sitting on top of the cabinet.

…The defrost system is hot gas, employing a timer to initiate the defrost cycle.

When you arrive to evaluate this equipment, you find all refrigerant

pressures to be normal and the box temperature is near 0-degrees. Satisfied that the

freezing mode of the unit is operating according to specifications, your next step is

to force a defrost mode.

When you do, you note that the evaporator fan motor shuts down and the hot

gas solenoid operates normally. Figure One shows you the path of refrigerant flow

during the hot gas defrost mode…from the got gas solenoid valve, directly through

the evaporator, and back to the compressor.

Near the end of the defrost mode, you find that the compressor kicks off on

overload, and isn‟t able to re-start until after an extended delay.

Your troubleshooting question: What mistake was made during the design

and construction of this system, causing the compressor to shut down?

TROUBLESHOOTING PROBLEM SOLUTIONS

Problem #1: A SPLIT SYSTEM THAT‟S NOT KEEPING THE BUILDING

COMFORTABLE

The next step is to access the extremely dirty indoor coil and clean it. The

maximum pressure drop on a wet coil indoor coil should never be more than 0.3

inches of water column (w.g. is the identifier we employ for water column inch

measurements) and a drop of 0.6 indicates a very high level of restriction in air

flow through the coil.

One aspect to keep in mind about this troubleshooting problem is that the

residence was identified as being a rental, which means that the tenants may not

have taken as much care as a homeowner would when it‟s time to changing a dirty

filter. In some cases, a dirty filter would simply be removed for a period of time,

allowing the unit to run without any filtering at all, as long as it was providing cool

air at the moment.

Problem #2: A HEAT PUMP THAT WON‟T COOL

The specific failure is related to the wiring modification by the previous

technician. Without the two run capacitor design and the trickle circuit through the

start winding that provided crankcase heat in the off cycle, liquid refrigerant

migrated to the compressor.

When this happens, the liquid refrigerant mixes with the oil in the

compressor crankcase, causing the oil to foam. This compromises the lubrication

process on start-up, and that results in a compressor failure. You can trace this

trickle circuit that allows a small amount of voltage to go through the start winding

in the off cycle, which creates a small amount of heat by tracing from L1, through

the Ca capacitor, through the start winding, through the OL overload, and back to

the L2 side of the line.

Problem #3: A ROOFTOP UNIT THAT WON‟T SHUT DOWN

The next step is to replace the Indoor Blower Delay Relay because the M1

and M2 contacts are stuck. When the call for heat from the thermostat was

terminated and the circuit was broken to the IBDR coil, the contacts should open to

shut the blower motor down after a delay.

Problem #4: A SECOND OPINION ON AN UNDERCOUNTER UNIT

You have confirmed the diagnosis of the first technician. A reading of

infinity from the C to R terminals proves an open winding, which explains why the

compressor would not start.

When testing compressor motor windings, your ohmmeter should read a given

resistance from C to R, from C to S, and from S to R. A fundamental rule of thumb

regarding single-phase compressor motor winding checks is that the start winding

will have a higher resistance than the run winding, and reading from S to R is the

total resistance of both windings.

Problem #5: A MOTEL ROOM HEAT PUMP THAT‟S NOT HEATING

The next step to take is to replace the defrost control. With no voltage being

delivered to the reversing valve, the unit was unable to provide heat.

Problem #6: A HEAT PUMP THAT‟S NOT COOLING PROPERLY

The problem component in this situation is the check valve located on the

liquid line leading to the indoor coil. It‟s stuck open. In the cooling mode, this

check valve is supposed to be closed, forcing the refrigerant through the metering

device.

When it‟s stuck open, no pressure drop can occur between the outdoor and

indoor coils because the refrigerant simply takes the path of least resistance

(through the open check valve and not through the metering device) to the coil as

we‟re showing in the accompanying illustration. The boiling point of the

refrigerant will not be reduced, resulting in no subcooling, higher than normal

suction pressure, and liquid flooding of the suction line, accumulator, and

compressor.

Problem #7: A SODA VENDING MACHINE THAT‟S NOT PERFORMING

PROPERLY…SOMETIMES

The next step to take is to replace the low-ambient control that is wired in

series with the condenser fan motor.

The control should be opening and breaking the circuit to the condenser fan

motor when the outdoor temperature drops, thereby maintaining the proper head

pressure. On the return trip during the low ambient condition, the condenser fan

motor was running, causing the high side pressure to drop too low in the later part

of the day.

Problem#8: A SPLIT SYSTEM THAT‟S NOT COOLING

The best correct answer to the first question is the transformer and the

blower motor. You could also say that the fan relay or the printed circuit board that

incorporates a fan relay assembly is OK too. All of these components can be

eliminated as possibilities because the indoor fan motor is running. The two correct

answers to the second question are the contactor and the condenser fan motor.

Without the contactor operating normally, the condenser fan motor wouldn‟t be

running, nor would the compressor be attempting to start. The final answer to our

“blowing warm air” complaint is that we would have to replace the potential relay.

When our meter showed infinity at the SR coil, it proved that the coil was open,

which would prevent the relay from opening the SR switch. And the end result of

that situation would be that the start capacitor would not be taken out of the circuit,

causing excessive current draw, which would cause the compressor to kick off on

its internal overload.

Problem #9: A 90-DEGREE OUTDOOR TEMPERATURE AND A CUSTOMER

THAT‟S HOT UNDER THE COLLAR

The problem with this unit is that it‟s low on refrigerant due to a leak. As far

as what has to be done in order to get it running properly again, we‟ll have to do

what the previous technician didn‟t do…locate the leak, recover the refrigerant,

make the repair, test for leaks with a nitrogen charge, then install a new drier,

evacuate to 500 microns, and recharge the system.

We were tipped off to the fact that the unit was low on refrigerant and cycling on

the low pressure switch when we consulted the wiring diagram. The partial

diagram in the accompanying illustration isolates the protection switches on the

control circuit…a low-pressure switch and a high-pressure switch.

Troubleshooting Problem #9 Partial Diagram

With the unit low on refrigerant, the low-pressure switch would open and break the

circuit to the contactor coil, thereby shutting the compressor and condenser fan

motor down. Since the metering device is a capillary tube, system equalization

occurs quickly, causing the low pressure to rise far enough to close the low-

pressure switch and re-start the compressor and condenser fan motor. The cycle

repeats as long as there is sufficient refrigerant in the system to cause an

equalization pressure that closes the low-pressure switch again.

Problem #10: A FOLLOW UP SERVICE CALL ON A THREE-PHASE UNIT

The error was switching two legs of the three-phase power supply when re-

connecting the wires to the disconnect switch assembly. This caused the indoor fan

motor to run counter-clockwise, rather than in the proper direction as shown in the

accompanying illustration.

Troubleshooting Problem #11: A REFRIGERATION SYSTEM WITH A

HISTORY OF COMPRESSOR FAILURE

The root of the problem in this situation was incorrect piping of the suction

line. Since this system does have a pump-down system, the piping should have

been accomplished in the manner we‟re showing in the accompanying illustration

rather than the way it was shown in our problem.

With the piping directly down to the compressor, refrigerant migration to the

crankcase occurred during off cycles, causing oil-foaming on start-up and eventual

mechanical failure of the compressor. Our system in this problem situation was

shown without a liquid line solenoid, which would be part of a pump-down system.

Problem #12: A HEAT PUMP WITH AN “AIR FLOW” PROBLEM

The failed component is the 4R control relay. We proved this when we read

24 volts at 4R, which represents the coil on our schematic (see the segmented

accompanying diagram), and when we also read 24 volts at the 4R2 contacts, and

additionally when we read 240 volts at the 4R1 contacts. These tests indicated that

although control voltage was applied to the coil, the contacts of the relay remained

open.

The reason the unit operated in the cooling mode is because the cooling circuit was

complete when the thermostat directly energized the contactor and fan relay coils.

In the heating mode, the 4R relay provides the circuit to the contactor and fan relay

coils.

Problem #13: AN ELECTRIC FURNACE THAT‟S NOT HEATING UP

The air is being only slightly heated due to only one element operating. The

component that needs replacing is the #1 sequencer. It‟s “A” contacts are not

closing to energize the control segment of sequencer #2, not allowing the next step

in the heating element operation sequence. Reading 24-volts across the A1 and A2

terminals proved the contacts were remaining open.

Problem #14: A GAS FURNACE THAT‟S NOT HEATING

The next step is to focus on the main gas valve itself. Testing at SEC2 and TP7

and getting a 24-volt reading during an ignition attempt proved that the board was

allowing 24-volts out to the gas valve coil, proving that the switching assembly

inside the board was not the problem.

A follow up test with a voltmeter would show 24-volts applied to the gas valve coil

providing there was no problem with the wiring harness. With 24-volts applied to

the main gas valve coil, it should be opening and allowing gas flow to the burner.

Problem #15: A HEAT PUMP THAT‟S NOT HEATING AT FULL CAPACITY

The repair we have to accomplish is the replacement of the 2H and 3H auxiliary

heaters.

In both cases, we measured 220-volts at the heating elements, but the current draw

was zero, indicating that while voltage was applied to the heating elements,

indicating that they were not operating. It‟s likely that one element had failed at

some point in the past, but the lack of heat capacity wasn‟t really noticed until the

second element failed.

Problem #16: A GAS FURNACE THAT‟S HEATING INTERMITTENTLY

The component that needs replacing is the indoor blower motor run capacitor. On

our diagram it shows the rating for 5.0 UF (UF stands for microfarad), and our

meter test showed the capacitor at a rating of only 2.5 microfarads.

Problem #17: SIX HOUSES WITH THE SAME COMPLAINT

The installation error that was committed was leaving the indoor blower

motor wired as it arrived from the factory for low speed operation. A temperature

rise test showed that the actual temperature rise was 70-degrees, while it was

supposed to be 65-degrees. This proved that the 4-speed blower motor should have

been set to operate on the medium-low speed rather than low.

Increasing the air flow by wiring the motor to a higher speed allowed for

more air flow in the duct system, and provided proper air volume into the back

bedroom.

Problem #18

THE EXCESSIVELY EXPENSIVE HEAT PUMP

The specific failure is the DFR (Defrost Relay). The normally closed

contacts are not opening, and the outdoor fan motor continues to run during

defrost, resulting in a longer-than-normal defrost time.

With the outdoor fan motor operating during defrost, it takes longer to heat

up the outdoor coil with hot vapor refrigerant, extending the defrost time and the

amount of time the supplemental heat strips are called on to temper the indoor air.

Problem #19

A NEARLY ANCIENT PACKAGE UNIT THAT‟S NOT COOLING LIKE IT

SHOULD

The component that needs to be replaced is the printed circuit board, due to

the failure of the CIFR that is an integrated part of the board assembly. The

contacts of the relay should be closed when 24-VAC is supplied to the coil.

However, we proved they were remaining open when we read 0-VAC at the motor

winding connections and 230-VAC across the relay contacts.

Problem #20

A SMALL WALK-IN FREEZER THAT‟S OPERATING ERRATICALLY

The fundamental answer to this problem is that there was no accumulator

installed on the suction line of the system, resulting in flood-back to the

compressor during the defrost mode. This caused the compressor to kick off on

overload, and the delay in a re-start resulted in excessive temperature rise inside

the box.

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